1. Field of the Invention
The present invention relates to a powder magnetic core, particularly to a powder magnetic core whose initial permeability is lowered, which therefore indicates a high permeability even with application of a high magnetic field, and which exerts a superior direct-current superimposition property as a result.
2. Prior Art
A powder magnetic core can be manufactured with a high yield, even when an object product has a small size and complicated shape. Therefore, the core has been started to be broadly used instead of a laminated magnetic core using a silicate steel plate, which is a mainstream of a conventional magnetic core. Specifically, the core is used, for example, in a core of a transformer for charging a battery mounted on an electric car or a hybrid car, an inductor to be used in an unstop power source (UPS), and the like.
The powder magnetic core is generally manufactured in a manner as follows.
First, a soft magnetic alloy having a predetermined composition is subjected to a mechanical grinding or an atomization process and a powder having a predetermined grain size distribution (hereinafter referred to as a soft magnetic powder) is manufactured.
Subsequently, the soft magnetic powder is homogeneously mixed with a predetermined amount of an insulation material and binder component. This treatment is performed in order to raise an electric resistivity of the powder magnetic core to be manufactured. Examples of the insulation material to be used in this case include: oxide powders such as an Al2O3 powder, and SiO2 powder; and nitride powders such as AlN, Si3N4, and BN. Moreover, examples of the binder component include water glass also having an electric insulation property, and organic polymers such as silicone resin.
Additionally, in the following description, the above-described insulation material and binder component will collectively be referred to as “an insulation bindery”.
Subsequently, the mixture is charged into a mold, and molded with a predetermined pressure so that a green compact of the powder magnetic core is manufactured. Additionally, in this case, to enhance a molding property, usually a predetermined amount of lubricants such as zinc stearate is further mixed into the above-described mixture.
Finally, the green compact is heat-treated, a molding strain accumulated during molding is released, and a targeted powder magnetic core is obtained.
The powder magnetic core manufactured in this manner, in general, as a direct-current magnetic field (applied magnetic field) intensifies, gradually increases its magnetic flux density and when the applied magnetic field reaches a certain intensity, its magnetic flux density is saturated. Such magnetization curve (B-H curve) is drawn.
Furthermore, the permeability in the magnetic field (differential specific permeability) is defined with a value obtained by superimposing an alternating-current micro magnetic field upon a certain direct-current magnetic field, slightly changing the magnetic field, and dividing an obtained change amount of the magnetic flux density by a micro change amount of the magnetic field in the process of the increase of the magnetic flux density. Therefore, when an inclination of the B-H curve is reduced, that is, when the applied magnetic field is intensified, the differential specific permeability is reduced. Therefore, the permeability decreases. When and after reaching saturation magnetization, the permeability substantially indicates 1.
Additionally, with the high-permeability powder magnetic core manufactured using soft magnetic powders such as a Sendust powder as a raw material, when the core is used by conduction of a large current, an intense direct-current magnetic field is applied to the core. Therefore, the magnetic flux density of the powder magnetic core rapidly approaches the saturation. As a result, the permeability decreases toward 1. That is, the powder magnetic core having such high permeability is inferior in the direct-current superimposition property.
Usually, in a use field of the powder magnetic core, the powder magnetic core whose initial permeability is about 60 to 125 in practically used. However, with the powder magnetic core having such initial permeability, for example, when a high magnetic field of 16 kA/m or more is applied, the permeability becomes remarkably low, which gives rise to a problem that the core cannot bear its practical use. Particularly, in recent years, the electric car, hybrid car, and the like have been driven with an increasingly large current. Accordingly, the magnetic field applied to the mounted core tends to increase. Therefore, there has been a demand for a capability of the powder magnetic core which can bear a large-current use.
Therefore, for example, even when a high magnetic field of 16 kA/m or more is applied, in order to suppress deterioration of the direct-current superimposition property in a state where a necessary level of permeability is secured, it is effective to lower the initial permeability of the powder magnetic core to be used.
Moreover, it is generally known that the permeability is a function of density of the powder magnetic core. That is, the powder magnetic core having a low density indicates a low permeability. In consideration of this, in order to achieve the above object of lowering the initial permeability of the powder magnetic core, it is effective to lower the density of the powder magnetic core.
However, even in this case, it should be considered that the powder magnetic core has a magnetic property that the magnetic flux density of the powder magnetic core increases as the applied magnetic field intensifies, and finally reaches saturation magnetization. Moreover, even if the initial permeability is low, a saturation magnetic flux density of the powder magnetic core has to satisfy the necessary level for practical use. Another point is that the core should be able to be manufactured industrially with high yield.